gecko-dev/xpfe/appshell/nsWindowMediator.cpp

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "nsCOMPtr.h"
#include "nsEnumeratorUtils.h"
#include "nsString.h"
#include "nsReadableUtils.h"
#include "nsUnicharUtils.h"
#include "nsTArray.h"
#include "nsIBaseWindow.h"
#include "nsIWidget.h"
#include "nsIObserverService.h"
#include "nsISimpleEnumerator.h"
#include "nsAppShellWindowEnumerator.h"
#include "nsWindowMediator.h"
#include "nsIWindowMediatorListener.h"
#include "nsGlobalWindow.h"
#include "nsServiceManagerUtils.h"

#include "nsIDocShell.h"
#include "nsIInterfaceRequestor.h"
#include "nsIInterfaceRequestorUtils.h"
#include "nsIAppWindow.h"

using namespace mozilla;

nsresult nsWindowMediator::GetDOMWindow(
    nsIAppWindow* inWindow, nsCOMPtr<nsPIDOMWindowOuter>& outDOMWindow) {
  nsCOMPtr<nsIDocShell> docShell;

  outDOMWindow = nullptr;
  inWindow->GetDocShell(getter_AddRefs(docShell));
  NS_ENSURE_TRUE(docShell, NS_ERROR_FAILURE);

  outDOMWindow = docShell->GetWindow();
  return outDOMWindow ? NS_OK : NS_ERROR_FAILURE;
}

nsWindowMediator::nsWindowMediator()
    : mEnumeratorList(),
      mOldestWindow(nullptr),
      mTopmostWindow(nullptr),
      mTimeStamp(0),
      mSortingZOrder(false),
      mReady(false) {}

nsWindowMediator::~nsWindowMediator() {
  while (mOldestWindow) UnregisterWindow(mOldestWindow);
}

nsresult nsWindowMediator::Init() {
  nsresult rv;
  nsCOMPtr<nsIObserverService> obsSvc =
      do_GetService("@mozilla.org/observer-service;1", &rv);
  NS_ENSURE_SUCCESS(rv, rv);
  rv = obsSvc->AddObserver(this, "xpcom-shutdown", true);
  NS_ENSURE_SUCCESS(rv, rv);

  mReady = true;
  return NS_OK;
}

NS_IMETHODIMP nsWindowMediator::RegisterWindow(nsIAppWindow* inWindow) {
  MOZ_RELEASE_ASSERT(NS_IsMainThread());

  if (!mReady) {
    NS_ERROR("Mediator is not initialized or about to die.");
    return NS_ERROR_FAILURE;
  }

  if (GetInfoFor(inWindow)) {
    NS_ERROR("multiple window registration");
    return NS_ERROR_FAILURE;
  }

  mTimeStamp++;

  // Create window info struct and add to list of windows
  nsWindowInfo* windowInfo = new nsWindowInfo(inWindow, mTimeStamp);

  for (const auto& listener : mListeners.ForwardRange()) {
    listener->OnOpenWindow(inWindow);
  }

  if (mOldestWindow)
    windowInfo->InsertAfter(mOldestWindow->mOlder, nullptr);
  else
    mOldestWindow = windowInfo;

  return NS_OK;
}

NS_IMETHODIMP
nsWindowMediator::UnregisterWindow(nsIAppWindow* inWindow) {
  MOZ_RELEASE_ASSERT(NS_IsMainThread());
  MOZ_ASSERT(mReady);
  NS_ENSURE_STATE(mReady);
  nsWindowInfo* info = GetInfoFor(inWindow);
  if (info) return UnregisterWindow(info);
  return NS_ERROR_INVALID_ARG;
}

nsresult nsWindowMediator::UnregisterWindow(nsWindowInfo* inInfo) {
  // Inform the iterators
  uint32_t index = 0;
  while (index < mEnumeratorList.Length()) {
    mEnumeratorList[index]->WindowRemoved(inInfo);
    index++;
  }

  nsIAppWindow* window = inInfo->mWindow.get();
  for (const auto& listener : mListeners.ForwardRange()) {
    listener->OnCloseWindow(window);
  }

  // Remove from the lists and free up
  if (inInfo == mOldestWindow) mOldestWindow = inInfo->mYounger;
  if (inInfo == mTopmostWindow) mTopmostWindow = inInfo->mLower;
  inInfo->Unlink(true, true);
  if (inInfo == mOldestWindow) mOldestWindow = nullptr;
  if (inInfo == mTopmostWindow) mTopmostWindow = nullptr;
  delete inInfo;

  return NS_OK;
}

nsWindowInfo* nsWindowMediator::GetInfoFor(nsIAppWindow* aWindow) {
  nsWindowInfo *info, *listEnd;

  if (!aWindow) return nullptr;

  info = mOldestWindow;
  listEnd = nullptr;
  while (info != listEnd) {
    if (info->mWindow.get() == aWindow) return info;
    info = info->mYounger;
    listEnd = mOldestWindow;
  }
  return nullptr;
}

nsWindowInfo* nsWindowMediator::GetInfoFor(nsIWidget* aWindow) {
  nsWindowInfo *info, *listEnd;

  if (!aWindow) return nullptr;

  info = mOldestWindow;
  listEnd = nullptr;

  nsCOMPtr<nsIWidget> scanWidget;
  while (info != listEnd) {
    nsCOMPtr<nsIBaseWindow> base(do_QueryInterface(info->mWindow));
    if (base) base->GetMainWidget(getter_AddRefs(scanWidget));
    if (aWindow == scanWidget.get()) return info;
    info = info->mYounger;
    listEnd = mOldestWindow;
  }
  return nullptr;
}

NS_IMETHODIMP
nsWindowMediator::GetEnumerator(const char16_t* inType,
                                nsISimpleEnumerator** outEnumerator) {
  MOZ_RELEASE_ASSERT(NS_IsMainThread());
  NS_ENSURE_ARG_POINTER(outEnumerator);
  if (!mReady) {
    // If we get here with mReady false, we most likely did observe
    // xpcom-shutdown. We will return an empty enumerator such that
    // we make happy Javascripts calling late without throwing.
    return NS_NewEmptyEnumerator(outEnumerator);
  }
  RefPtr<nsAppShellWindowEnumerator> enumerator =
      new nsASDOMWindowEarlyToLateEnumerator(inType, *this);
  enumerator.forget(outEnumerator);
  return NS_OK;
}

NS_IMETHODIMP
nsWindowMediator::GetAppWindowEnumerator(const char16_t* inType,
                                         nsISimpleEnumerator** outEnumerator) {
  MOZ_RELEASE_ASSERT(NS_IsMainThread());
  NS_ENSURE_ARG_POINTER(outEnumerator);
  if (!mReady) {
    // If we get here with mReady false, we most likely did observe
    // xpcom-shutdown. We will return an empty enumerator such that
    // we make happy Javascripts calling late without throwing.
    return NS_NewEmptyEnumerator(outEnumerator);
  }
  RefPtr<nsAppShellWindowEnumerator> enumerator =
      new nsASAppWindowEarlyToLateEnumerator(inType, *this);
  enumerator.forget(outEnumerator);
  return NS_OK;
}

NS_IMETHODIMP
nsWindowMediator::GetZOrderAppWindowEnumerator(const char16_t* aWindowType,
                                               bool aFrontToBack,
                                               nsISimpleEnumerator** _retval) {
  MOZ_RELEASE_ASSERT(NS_IsMainThread());
  NS_ENSURE_ARG_POINTER(_retval);
  if (!mReady) {
    // If we get here with mReady false, we most likely did observe
    // xpcom-shutdown. We will return an empty enumerator such that
    // we make happy Javascripts calling late without throwing.
    return NS_NewEmptyEnumerator(_retval);
  }
  RefPtr<nsAppShellWindowEnumerator> enumerator;
  if (aFrontToBack)
    enumerator = new nsASAppWindowFrontToBackEnumerator(aWindowType, *this);
  else
    enumerator = new nsASAppWindowBackToFrontEnumerator(aWindowType, *this);

  enumerator.forget(_retval);
  return NS_OK;
}

void nsWindowMediator::AddEnumerator(nsAppShellWindowEnumerator* inEnumerator) {
  mEnumeratorList.AppendElement(inEnumerator);
}

int32_t nsWindowMediator::RemoveEnumerator(
    nsAppShellWindowEnumerator* inEnumerator) {
  return mEnumeratorList.RemoveElement(inEnumerator);
}

// Returns the window of type inType ( if null return any window type ) which
// has the most recent time stamp
NS_IMETHODIMP
nsWindowMediator::GetMostRecentWindow(const char16_t* inType,
                                      mozIDOMWindowProxy** outWindow) {
  MOZ_RELEASE_ASSERT(NS_IsMainThread());
  NS_ENSURE_ARG_POINTER(outWindow);
  *outWindow = nullptr;
  if (!mReady) return NS_OK;

  // Find the most window with the highest time stamp that matches
  // the requested type
  nsWindowInfo* info = MostRecentWindowInfo(inType, false);
  if (info && info->mWindow) {
    nsCOMPtr<nsPIDOMWindowOuter> DOMWindow;
    if (NS_SUCCEEDED(GetDOMWindow(info->mWindow, DOMWindow))) {
      DOMWindow.forget(outWindow);
      return NS_OK;
    }
    return NS_ERROR_FAILURE;
  }

  return NS_OK;
}

NS_IMETHODIMP
nsWindowMediator::GetMostRecentBrowserWindow(mozIDOMWindowProxy** outWindow) {
  nsresult rv = GetMostRecentWindow(u"navigator:browser", outWindow);
  NS_ENSURE_SUCCESS(rv, rv);

#ifdef MOZ_WIDGET_ANDROID
  if (!*outWindow) {
    rv = GetMostRecentWindow(u"navigator:geckoview", outWindow);
    NS_ENSURE_SUCCESS(rv, rv);
  }
#endif

#ifdef MOZ_THUNDERBIRD
  if (!*outWindow) {
    rv = GetMostRecentWindow(u"mail:3pane", outWindow);
    NS_ENSURE_SUCCESS(rv, rv);
  }
#endif

  return NS_OK;
}

NS_IMETHODIMP
nsWindowMediator::GetMostRecentNonPBWindow(const char16_t* aType,
                                           mozIDOMWindowProxy** aWindow) {
  MOZ_RELEASE_ASSERT(NS_IsMainThread());
  NS_ENSURE_ARG_POINTER(aWindow);
  *aWindow = nullptr;

  nsWindowInfo* info = MostRecentWindowInfo(aType, true);
  nsCOMPtr<nsPIDOMWindowOuter> domWindow;
  if (info && info->mWindow) {
    GetDOMWindow(info->mWindow, domWindow);
  }

  if (!domWindow) {
    return NS_ERROR_FAILURE;
  }

  domWindow.forget(aWindow);
  return NS_OK;
}

nsWindowInfo* nsWindowMediator::MostRecentWindowInfo(
    const char16_t* inType, bool aSkipPrivateBrowsingOrClosed) {
  int32_t lastTimeStamp = -1;
  nsAutoString typeString(inType);
  bool allWindows = !inType || typeString.IsEmpty();

  // Find the most recent window with the highest time stamp that matches
  // the requested type and has the correct browsing mode.
  nsWindowInfo* searchInfo = mOldestWindow;
  nsWindowInfo* listEnd = nullptr;
  nsWindowInfo* foundInfo = nullptr;
  for (; searchInfo != listEnd; searchInfo = searchInfo->mYounger) {
    listEnd = mOldestWindow;

    if (!allWindows && !searchInfo->TypeEquals(typeString)) {
      continue;
    }
    if (searchInfo->mTimeStamp < lastTimeStamp) {
      continue;
    }
    if (!searchInfo->mWindow) {
      continue;
    }
    if (aSkipPrivateBrowsingOrClosed) {
      nsCOMPtr<nsIDocShell> docShell;
      searchInfo->mWindow->GetDocShell(getter_AddRefs(docShell));
      nsCOMPtr<nsILoadContext> loadContext = do_QueryInterface(docShell);
      if (!loadContext || loadContext->UsePrivateBrowsing()) {
        continue;
      }

      nsCOMPtr<nsPIDOMWindowOuter> piwindow = docShell->GetWindow();
      if (!piwindow || piwindow->Closed()) {
        continue;
      }
    }

    foundInfo = searchInfo;
    lastTimeStamp = searchInfo->mTimeStamp;
  }

  return foundInfo;
}

NS_IMETHODIMP
nsWindowMediator::GetOuterWindowWithId(uint64_t aWindowID,
                                       mozIDOMWindowProxy** aWindow) {
  RefPtr<nsGlobalWindowOuter> window =
      nsGlobalWindowOuter::GetOuterWindowWithId(aWindowID);
  window.forget(aWindow);
  return NS_OK;
}

NS_IMETHODIMP
nsWindowMediator::GetCurrentInnerWindowWithId(uint64_t aWindowID,
                                              mozIDOMWindow** aWindow) {
  RefPtr<nsGlobalWindowInner> window =
      nsGlobalWindowInner::GetInnerWindowWithId(aWindowID);

  // not found
  if (!window) return NS_OK;

  nsCOMPtr<nsPIDOMWindowOuter> outer = window->GetOuterWindow();
  NS_ENSURE_TRUE(outer, NS_ERROR_UNEXPECTED);

  // outer is already using another inner, so it's same as not found
  if (outer->GetCurrentInnerWindow() != window) return NS_OK;

  window.forget(aWindow);
  return NS_OK;
}

NS_IMETHODIMP
nsWindowMediator::UpdateWindowTimeStamp(nsIAppWindow* inWindow) {
  MOZ_RELEASE_ASSERT(NS_IsMainThread());
  MOZ_ASSERT(mReady);
  NS_ENSURE_STATE(mReady);
  nsWindowInfo* info = GetInfoFor(inWindow);
  if (info) {
    // increment the window's time stamp
    info->mTimeStamp = ++mTimeStamp;
    return NS_OK;
  }
  return NS_ERROR_FAILURE;
}

/* This method's plan is to intervene only when absolutely necessary.
   We will get requests to place our windows behind unknown windows.
   For the most part, we need to leave those alone (turning them into
   explicit requests to be on top breaks Windows.) So generally we
   calculate a change as seldom as possible.
*/
NS_IMETHODIMP
nsWindowMediator::CalculateZPosition(nsIAppWindow* inWindow,
                                     uint32_t inPosition, nsIWidget* inBelow,
                                     uint32_t* outPosition,
                                     nsIWidget** outBelow, bool* outAltered) {
  MOZ_RELEASE_ASSERT(NS_IsMainThread());
  NS_ENSURE_ARG_POINTER(outBelow);
  MOZ_ASSERT(mReady);
  NS_ENSURE_STATE(mReady);

  *outBelow = nullptr;

  if (!inWindow || !outPosition || !outAltered) return NS_ERROR_NULL_POINTER;

  if (inPosition != nsIWindowMediator::zLevelTop &&
      inPosition != nsIWindowMediator::zLevelBottom &&
      inPosition != nsIWindowMediator::zLevelBelow)
    return NS_ERROR_INVALID_ARG;

  nsWindowInfo* info = mTopmostWindow;
  nsIAppWindow* belowWindow = nullptr;
  bool found = false;
  nsresult result = NS_OK;

  *outPosition = inPosition;
  *outAltered = false;

  if (mSortingZOrder) {  // don't fight SortZOrder()
    *outBelow = inBelow;
    NS_IF_ADDREF(*outBelow);
    return NS_OK;
  }

  uint32_t inZ;
  GetZLevel(inWindow, &inZ);

  if (inPosition == nsIWindowMediator::zLevelBelow) {
    // locate inBelow. use topmost if it can't be found or isn't in the
    // z-order list
    info = GetInfoFor(inBelow);
    if (!info || (info->mYounger != info && info->mLower == info))
      info = mTopmostWindow;
    else
      found = true;

    if (!found) {
      /* Treat unknown windows as a request to be on top.
         Not as it should be, but that's what Windows gives us.
         Note we change inPosition, but not *outPosition. This forces
         us to go through the "on top" calculation just below, without
         necessarily changing the output parameters. */
      inPosition = nsIWindowMediator::zLevelTop;
    }
  }

  if (inPosition == nsIWindowMediator::zLevelTop) {
    if (mTopmostWindow && mTopmostWindow->mZLevel > inZ) {
      // asked for topmost, can't have it. locate highest allowed position.
      do {
        if (info->mZLevel <= inZ) break;
        info = info->mLower;
      } while (info != mTopmostWindow);

      *outPosition = nsIWindowMediator::zLevelBelow;
      belowWindow = info->mHigher->mWindow;
      *outAltered = true;
    }
  } else if (inPosition == nsIWindowMediator::zLevelBottom) {
    if (mTopmostWindow && mTopmostWindow->mHigher->mZLevel < inZ) {
      // asked for bottommost, can't have it. locate lowest allowed position.
      do {
        info = info->mHigher;
        if (info->mZLevel >= inZ) break;
      } while (info != mTopmostWindow);

      *outPosition = nsIWindowMediator::zLevelBelow;
      belowWindow = info->mWindow;
      *outAltered = true;
    }
  } else {
    unsigned long relativeZ;

    // check that we're in the right z-plane
    if (found) {
      belowWindow = info->mWindow;
      relativeZ = info->mZLevel;
      if (relativeZ > inZ) {
        // might be OK. is lower window, if any, lower?
        if (info->mLower != info && info->mLower->mZLevel > inZ) {
          do {
            if (info->mZLevel <= inZ) break;
            info = info->mLower;
          } while (info != mTopmostWindow);

          belowWindow = info->mHigher->mWindow;
          *outAltered = true;
        }
      } else if (relativeZ < inZ) {
        // nope. look for a higher window to be behind.
        do {
          info = info->mHigher;
          if (info->mZLevel >= inZ) break;
        } while (info != mTopmostWindow);

        if (info->mZLevel >= inZ)
          belowWindow = info->mWindow;
        else
          *outPosition = nsIWindowMediator::zLevelTop;
        *outAltered = true;
      }  // else they're equal, so it's OK
    }
  }

  if (NS_SUCCEEDED(result) && belowWindow) {
    nsCOMPtr<nsIBaseWindow> base(do_QueryInterface(belowWindow));
    if (base)
      base->GetMainWidget(outBelow);
    else
      result = NS_ERROR_NO_INTERFACE;
  }

  return result;
}

NS_IMETHODIMP
nsWindowMediator::SetZPosition(nsIAppWindow* inWindow, uint32_t inPosition,
                               nsIAppWindow* inBelow) {
  MOZ_RELEASE_ASSERT(NS_IsMainThread());
  nsWindowInfo *inInfo, *belowInfo;

  if ((inPosition != nsIWindowMediator::zLevelTop &&
       inPosition != nsIWindowMediator::zLevelBottom &&
       inPosition != nsIWindowMediator::zLevelBelow) ||
      !inWindow) {
    return NS_ERROR_INVALID_ARG;
  }

  if (mSortingZOrder)  // don't fight SortZOrder()
    return NS_OK;

  MOZ_ASSERT(mReady);
  NS_ENSURE_STATE(mReady);

  /* Locate inWindow and unlink it from the z-order list.
     It's important we look for it in the age list, not the z-order list.
     This is because the former is guaranteed complete, while
     now may be this window's first exposure to the latter. */
  inInfo = GetInfoFor(inWindow);
  if (!inInfo) return NS_ERROR_INVALID_ARG;

  // locate inBelow, place inWindow behind it
  if (inPosition == nsIWindowMediator::zLevelBelow) {
    belowInfo = GetInfoFor(inBelow);
    // it had better also be in the z-order list
    if (belowInfo && belowInfo->mYounger != belowInfo &&
        belowInfo->mLower == belowInfo) {
      belowInfo = nullptr;
    }
    if (!belowInfo) {
      if (inBelow)
        return NS_ERROR_INVALID_ARG;
      else
        inPosition = nsIWindowMediator::zLevelTop;
    }
  }
  if (inPosition == nsIWindowMediator::zLevelTop ||
      inPosition == nsIWindowMediator::zLevelBottom)
    belowInfo = mTopmostWindow ? mTopmostWindow->mHigher : nullptr;

  if (inInfo != belowInfo) {
    inInfo->Unlink(false, true);
    inInfo->InsertAfter(nullptr, belowInfo);
  }
  if (inPosition == nsIWindowMediator::zLevelTop) mTopmostWindow = inInfo;

  return NS_OK;
}

NS_IMETHODIMP
nsWindowMediator::GetZLevel(nsIAppWindow* aWindow, uint32_t* _retval) {
  NS_ENSURE_ARG_POINTER(_retval);
  *_retval = nsIAppWindow::normalZ;
  // This can fail during window destruction.
  nsWindowInfo* info = GetInfoFor(aWindow);
  if (info) {
    *_retval = info->mZLevel;
  }
  return NS_OK;
}

NS_IMETHODIMP
nsWindowMediator::SetZLevel(nsIAppWindow* aWindow, uint32_t aZLevel) {
  MOZ_RELEASE_ASSERT(NS_IsMainThread());
  MOZ_ASSERT(mReady);
  NS_ENSURE_STATE(mReady);

  nsWindowInfo* info = GetInfoFor(aWindow);
  NS_ASSERTION(info, "setting z level of unregistered window");
  if (!info) return NS_ERROR_FAILURE;

  if (info->mZLevel != aZLevel) {
    bool lowered = info->mZLevel > aZLevel;
    info->mZLevel = aZLevel;
    if (lowered)
      SortZOrderFrontToBack();
    else
      SortZOrderBackToFront();
  }
  return NS_OK;
}

/*   Fix potentially out-of-order windows by performing an insertion sort
   on the z-order list. The method will work no matter how broken the
   list, but its assumed usage is immediately after one window's z level
   has been changed, so one window is potentially out of place. Such a sort
   is most efficiently done in a particular direction. Use this one
   if a window's z level has just been reduced, so the sort is most efficiently
   done front to back.
     Note it's hardly worth going to all the trouble to write two versions
   of this method except that if we choose the inefficient sorting direction,
   on slow systems windows could visibly bubble around the window that
   was moved.
*/
void nsWindowMediator::SortZOrderFrontToBack() {
  nsWindowInfo *scan,  // scans list looking for problems
      *search,         // searches for correct placement for scan window
      *prev,           // previous search element
      *lowest;         // bottom-most window in list
  bool finished;

  if (!mTopmostWindow)  // early during program execution there's no z list yet
    return;  // there's also only one window, so this is not dangerous

  mSortingZOrder = true;

  /* Step through the list from top to bottom. If we find a window which
     should be moved down in the list, move it to its highest legal position. */
  do {
    finished = true;
    lowest = mTopmostWindow->mHigher;
    scan = mTopmostWindow;
    while (scan != lowest) {
      uint32_t scanZ = scan->mZLevel;
      if (scanZ < scan->mLower->mZLevel) {  // out of order
        search = scan->mLower;
        do {
          prev = search;
          search = search->mLower;
        } while (prev != lowest && scanZ < search->mZLevel);

        // reposition |scan| within the list
        if (scan == mTopmostWindow) mTopmostWindow = scan->mLower;
        scan->Unlink(false, true);
        scan->InsertAfter(nullptr, prev);

        // fix actual window order
        nsCOMPtr<nsIBaseWindow> base;
        nsCOMPtr<nsIWidget> scanWidget;
        nsCOMPtr<nsIWidget> prevWidget;
        base = do_QueryInterface(scan->mWindow);
        if (base) base->GetMainWidget(getter_AddRefs(scanWidget));
        base = do_QueryInterface(prev->mWindow);
        if (base) base->GetMainWidget(getter_AddRefs(prevWidget));
        if (scanWidget)
          scanWidget->PlaceBehind(eZPlacementBelow, prevWidget, false);

        finished = false;
        break;
      }
      scan = scan->mLower;
    }
  } while (!finished);

  mSortingZOrder = false;
}

// see comment for SortZOrderFrontToBack
void nsWindowMediator::SortZOrderBackToFront() {
  nsWindowInfo *scan,  // scans list looking for problems
      *search,         // searches for correct placement for scan window
      *lowest;         // bottom-most window in list
  bool finished;

  if (!mTopmostWindow)  // early during program execution there's no z list yet
    return;  // there's also only one window, so this is not dangerous

  mSortingZOrder = true;

  /* Step through the list from bottom to top. If we find a window which
     should be moved up in the list, move it to its lowest legal position. */
  do {
    finished = true;
    lowest = mTopmostWindow->mHigher;
    scan = lowest;
    while (scan != mTopmostWindow) {
      uint32_t scanZ = scan->mZLevel;
      if (scanZ > scan->mHigher->mZLevel) {  // out of order
        search = scan;
        do {
          search = search->mHigher;
        } while (search != lowest && scanZ > search->mZLevel);

        // reposition |scan| within the list
        if (scan != search && scan != search->mLower) {
          scan->Unlink(false, true);
          scan->InsertAfter(nullptr, search);
        }
        if (search == lowest) mTopmostWindow = scan;

        // fix actual window order
        nsCOMPtr<nsIBaseWindow> base;
        nsCOMPtr<nsIWidget> scanWidget;
        nsCOMPtr<nsIWidget> searchWidget;
        base = do_QueryInterface(scan->mWindow);
        if (base) base->GetMainWidget(getter_AddRefs(scanWidget));
        if (mTopmostWindow != scan) {
          base = do_QueryInterface(search->mWindow);
          if (base) base->GetMainWidget(getter_AddRefs(searchWidget));
        }
        if (scanWidget)
          scanWidget->PlaceBehind(eZPlacementBelow, searchWidget, false);
        finished = false;
        break;
      }
      scan = scan->mHigher;
    }
  } while (!finished);

  mSortingZOrder = false;
}

NS_IMPL_ISUPPORTS(nsWindowMediator, nsIWindowMediator, nsIObserver,
                  nsISupportsWeakReference)

NS_IMETHODIMP
nsWindowMediator::AddListener(nsIWindowMediatorListener* aListener) {
  NS_ENSURE_ARG_POINTER(aListener);

  mListeners.AppendElement(aListener);

  return NS_OK;
}

NS_IMETHODIMP
nsWindowMediator::RemoveListener(nsIWindowMediatorListener* aListener) {
  NS_ENSURE_ARG_POINTER(aListener);

  mListeners.RemoveElement(aListener);

  return NS_OK;
}

NS_IMETHODIMP
nsWindowMediator::Observe(nsISupports* aSubject, const char* aTopic,
                          const char16_t* aData) {
  if (!strcmp(aTopic, "xpcom-shutdown") && mReady) {
    MOZ_RELEASE_ASSERT(NS_IsMainThread());
    while (mOldestWindow) UnregisterWindow(mOldestWindow);
    mReady = false;
  }
  return NS_OK;
}